Documentation and renaming of "blankies" into somethign a bit easier to find.

* Rename SPlitExpression* to `SbtParser` denoting that is parses .sbt files
* Adds a few todos
* Document APIs for internal usage.
Merge pull request #1637 from sbt/wip/no-blankies-jsuereth

Documentation and renaming of "blankies" into somethign a bit easier to ...